I think it depends on the situation. Is he tanking it because he wants a better draft pick next year or is it because he is out of the playoff hunt and this is the only way he can keep it fun for himself?
Unfortunately, most basic leagues don't have any incentive for teams to try to win once it is clear they can't make the playoffs. Quite the contrary, many leagues have parity rules that give you draft priority next year for being bad this year. This is actually disincentive to try to win. The fact is, a bad team is a bad team, whether the guy made it that way on purpose or not.
You might think about trying to come up with a way to keep people interested for the entire year. Check the link below for some suggestions...
http://fantasyfootballcafe.com/forums/v ... p?t=204414